Bab and Bahá did not Reveal
everything, so another Prophet will come
soon
after them

Bahá'u'lláh, as it
was recorded in the writings of Nabil, Taherzadeh, and Shoghi Effendi, was
constantly receiving revelation from God in which He had to destroy . . .
the Plan of God, was not yet finished. In the writings of Shoghi
Effendi we find confirmation of this:

"As to those verses which He either dictated or wrote Himself, their
number was no less remarkable than either the wealth of material they
contained, or the diversity of subjects to which they referred.
A vast, and indeed the greater proportion of these Writings were,
alas, lost irretrievably to posterity. No less an authority than
Mirza Aqa Jan, Bahá'u'lláh's amanuensis, affirms, as reported by
Nabil, that by the express order of Bahá'u'lláh, hundreds of thousands
of verses, mostly written by His own hand, were obliterated and cast
into the river. 'Finding me reluctant to execute His orders,'
Mirza Aqa Jan has related to Nabil, 'Bahá'u'lláh would reassure me
saying: 'None is to be found at this time worthy to hear these
melodies.' Not once, or twice, but innumerable times, was I
commanded to repeat this act.'" (God Passes By, 138)